"Extreme" can be meant to describe any of the following musical elements: instrumentation (whether it is intended to be faster, more aggressive, abrasive or "heavier" than other metal styles), lyrics (dealing with darker, more sensational topics and themes), vocals (which often use guttural, harsh or abrasive singing), or appearance and stage demeanor (using corpse paint, Satanic or occult imagery). The "extreme" label is most commonly applied to bands whose music is extreme; for example, few would consider Kiss or Alice Cooper to be extreme metal, though they could be considered to employ "extreme" elements in their appearance and stage demeanor for their time.

According to ethnographerKeith Kahn-Harris,[2] the defining characteristics of extreme metal can all be regarded as clearly transgressive: the "extreme" traits noted above are all intended to violate or transgress given cultural, artistic, social or aesthetic boundaries.

Given the vagueness of existing definitions and considering the limitations such definitions have, there are many artists for whom the usage of the term "extreme metal" is a subject of debate.[2] However, Kahn-Harris also notes that many musicians and fans see such debates over style and genre as useless and unnecessary, or at least as given undue attention.